Constellation Constraint-Based Superposed 32QAM Scheme for NOMA System

In this letter, an odd-order superposed modulation scheme is studied in non-orthogonal multiple access (NOMA) system. Firstly, we design a novel superposed 32-quadrature amplitude modulation (QAM) constellation superposed by a novel 8QAM and quadrature phase shift keying (QPSK), where joint coding between QPSK and 8QAM is performed for 8QAM signal rotation. Then, a simplified optimal power allocation algorithm is given by maximizing the constellation constrained (CC) capacity at high SNR for obtaining optimal superposed constellation. Comparing with the superposed 32-phase-shift keying (PSK) scheme, our proposed scheme employing successive interference cancellation (SIC) for detection has a lower decoding complexity. Moreover, a wider power coefficient ratio range at the threshold of mutual information (MI) greater than 4.98 is achieved by our proposed scheme. It also demonstrates that the MI and BER performances of proposed scheme outperform than those of the other three schemes when the power coefficient ratio is optimal at high signal-to-noise ratio (SNR).